No single logging tool or measurement technique is presently available that can correctly identify properties of the rock formation, such as its porosity, in all circumstances. For example, bulk density measurements can only be used if the density of the rock matrix is known; sonic transit time measurements may be used if the transit times both for the rock and the fluids are known. A number of techniques, referred to as "crossplot techniques", exist that compare different measurements to estimate formation porosity in situ. However, none of these techniques is truly independent of the rock's geological make-up.
Another approach frequently used during open-hole logging is to perform bulk conductivity measurements in order to identify and separate oil-bearing zones which have low conductivity, from water-bearing zones which have high conductivity. However, in practice the interpretation of measurement data is typically obscured by the presence of highly-conductive clay attached to or interspersed with sand grains. Due to the fact that no simple measurement exists for generating quantitative in situ estimates of the amount of clay and the water volume bound to the clay, the interpretation of in situ conductivity data is still more of an art than a science.
The amount and type of clay in a formation is interesting to reservoir and to production engineers in its own right. For example, swelling and/or dislodging of certain clay particles may clog an otherwise permeable sand. Conventional logging tools have been often characterized in terms of their response to clay minerals and/or clay-bound water. In fact, most conventional logging measurements (such as neutron-absorption cross section, bulk density, natural gamma-ray radiation, spontaneous electric potential, sonic wave transit time, photoelectric absorption factor, etc.) respond in a qualitative way to the presence of clay in the formation being investigated, mostly because clays tend to accumulate heavy minerals. More information is contained in D. V. Ellis, "Well Logging For Earth Scientists," Elsevier 1987, chapter 19: "Clay Typing and Quantification from Logs," which chapter is incorporated herein by reference. Still, no single reliable method exists currently for estimating the parameters of the clay present in a formation. FIG. 1 shows the standard rock porosity model which provides an illustration of the issues discussed above. In particular, as shown in FIG. 1, the total porosity space is occupied by water and hydrocarbons. The volume excluded from what is designated in the figure as "effective porosity" is the clay-bound water fraction.
It is well known that the signal measured by NMR logging tools is proportional to the mean density of hydrogen nuclei in the fluid that occupies the pore space. Pulsed NMR measurements performed downhole are sensitive to the amount of hydrogen atoms from liquid or gaseous materials, but not from solid-state rock. Therefore, in principle, NMR is a truly lithology-independent porosity measurement. However, with reference to FIG. 1, current logging tools register only part of the total porosity of the formation because hydrogen nuclei in the rock matrix and those associated with clay particles relax too rapidly to be detected and measured under the limited signal-to-noise (SNR) conditions available downhole.
Accordingly, it is clear that the difference between a "total porosity" measurement (derived, for example, from a bulk density measurement, neutron absorption and/or sonic transit time) and the NMR-measured porosity can be interpreted as the amount of clay-bound water. See for example the disclosure in U.S. Pat. No. 5,557,200 assigned to the assignee of the present application, which is hereby incorporated by reference for all purposes. However, prior art methods require the use of separate techniques to measure the total porosity of a formation. In fact, obtaining an accurate estimate of this total porosity is still relatively difficult. Furthermore, an NMR measurement itself can be depressed by fluid effects, such as deficient hydrogen index, long polarization times T1, etc.
It has been recognized in the past that specific applications of NMR logging can be performed with less than full recovery of magnetization. For example, U.S. Pat. No. 5,389,877 to Sezginer et al. describes a method by which a moving NMR logging tool is used to quantify the amount of capillary-bound fluid volume BFV. However, in the patented method the clay-bound volume is not recorded, nor is the log interpretation improved. The patent merely records a sub-set of the data required for interpreting an NMR log. In particular, it requires that other logging tools provide an estimate of total porosity of the formation.
The method of the present invention, described in greater detail below, uses prior art logging tools and measurement apparatuses to obtain previously unavailable data relating to the composition of a geologic structure. In particular, a novel pulse sequence, signal processing technique and a method of interpretation of NMR measurements are proposed and used to obtain in a single experiment characteristics of the formation including its total porosity and clay mineral content which may then be used to determine additional key petrophysical parameters. In addition, the method of the present invention can also be used to measure properties of various porous materials having a fluid state.
Additional references which provide further background information include:
1. Ellis, D. V.: Well Logging for Earth Scientists, Elsevier, New York, N.Y. (1987) 305. PA0 2. Miller, M. N. et al.: "Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination," paper SPE 20561 presented at the 1990 SPE Annual Technical Conference and Exhibition, Proceedings, 321. PA0 3. Morriss, C. E. et al.: "Field Test of an Experimental Pulsed Nuclear Magnetism Tool," paper GGG presented at the 1993 Annual Logging Symposium of the Society of Professional Well Log Analysts. PA0 4. Chandler, R. N. et al.: "Improved Log Quality With a Dual-Frequency Pulsed NMR Tool," paper SPE 28365 presented at the 1994 SPE Annual Technical Conference and Exhibition, Proceedings, 23. PA0 5. Ellis, D. V.: Well Logging for Earth Scientists, Elsevier, New York, N.Y. (1987) 439-469. PA0 6. Korringa, J., Seevers, D. O. and Torrey, H. C.: "Theory of Spin Pumping and Relaxation in Systems With a Low Concentration of Electron Spin Resonance Centers," Phys. Rev. 127 (1962) 1143. PA0 7. Fripiat, J et al.: "Thermodynamic and Microdynamic Behavior of Water in Clay Suspensions and Gels," J. Colloid. Interface Sci. 89 (1982) 378. PA0 8. Woessner, D. E.: "An NMR Investigation Into The Range of the Surface Effect on the Rotation of Water Molecules," J. Magn. Reson. 39 (1980) 297. PA0 9. Prammer, M. G.: "NMR Pore Size Distributions and Permeability at The Well Site," paper SPE 28368 presented at the 1994 SPE Annual Technical Conference and Exhibition, Proceedings, 55. PA0 10. Freedman, R. and Morriss, C. E.: "Processing of Data From an NMR Logging Tool," paper SPE 30560 presented at the 1995 SPE Annual Technical Conference and Exhibition, Proceedings, 301. PA0 11. Prammer, M. G. et al.: "Lithology-Independent Gas Detection by Gradient-NMR Logging," paper SPE 30562 presented at the 1995 SPE Annual Technical Conference and Exhibition, Proceedings, 325. PA0 12. van Olphen, H. and Fripiat, J. J.: Data Handbook for Clay Minerals and Other Non-Metallic Minerals, Pergamon Press, New York, N.Y. (1979). PA0 13. Hower, J. and Mowatt, T. C.: "The Mineralogy of Illites and Mixed-Layer Illite Montmorillonites," The American Mineralogist, 51, (May-June 1966) 825.